One of the upsides of OS X Mountain Lion's already low purchase price of $20 is that one copy can be re-downloaded and installed on any other Macs that you own for free. The downside, of course, is that if another Mac in the household doesn't share the same Apple ID as you, the buyer, you have to get another copy. As it turns out, there's a very simple solution so you can just reinstall from the same copy as many times as you need to.

Aside from making an external installer and using that, there's a simple workaround to officially download Mountain Lion and install it the way Apple intended. Just follow these steps:

Open System Preferences on the Mac you want to update, then go to the Users & Groups section.

Press the + button to add a new user, and create an administrator account. (Don't worry, you can delete it later.)

Switch users and sign into this new account.

Open the Mac App Store and sign in with the Apple ID you used to purchase Mountain Lion.

Download Mountain Lion and install it.

Delete the new user account.

That's all there is to it. While you could technically use this technique to install Mountain Lion on the Macs of everyone you know, you shouldn't go that far. We're posting this tip to give you a way to install Mountain Lion on household Macs that may be in use by various people who don't share the same Apple ID. At a very reasonable $20, it's a negligible price to pay for a nice upgrade. That said, you're supposed to be able to install it on all your Macs. If you can't because of varied Apple IDs, use this method to get around the problem.